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Biodiesel, Water, and the Diesel Bug

As we all now will need to use white (tax paid) diesel in our boats, slowly diluting the variously coloured fuel we still have in our tanks, it is worth our while to take a look at what it is we will be purchasing. All white diesel now contains a percentage of biodiesel – usually around 5-7%. Pumps marked “B7” have 7% bio diesel and 93% petroleum diesel or petrodiesel.

Common biofuels:
E5 = 5% Ethanol added to the gasoline (petrol).
B7 = 7% Biodiesel added to the diesel

What is Biodiesel?

Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl (methyl, ethyl, or propyl) esters. Biodiesel is typically made by chemically reacting lipids (e.g., vegetable oil, soybean oil, animal fat (tallow)) with an alcohol (methanol) producing fatty acid esters.

Material Compatibility of Biodiesel

Biodiesel is compatible with most modern materials in your engine, tanks, valves, pipes, and hoses.

·         Plastics: Polyvinyl chloride (PVC) is slowly degraded by biodiesel. High-density polyethylene (HDPE) is compatible with biodiesel.

·         Metals: Biodiesel (like methanol) has an effect on copper-based materials (e.g. brass), and it also affects zinc, tin, lead, and cast iron. (Copper and zinc also promote oxidation, which can promote diesel bug – see below) Stainless steel (316 and 304) and aluminium are unaffected by biodiesel.

·         Rubber: Biodiesel affects types of natural rubbers found in some older engine components. Commonly used synthetic rubbers FKM- GBL-S and FKM- GF-S found in modern engines can handle biodiesel in all conditions

Diesel Contamination by Water

Water can get into diesel fuel as a result of condensation, rainwater penetration or adsorption from the air — modern biodiesel is especially hygroscopic. Water reduces the heat of fuel combustion, causing smoke, harder starting, and reduced power. It causes corrosion of fuel system components (pumps, fuel lines, etc.). Water freezes to form ice crystals that provide sites for nucleation, accelerating gelling of the fuel. And it causes pitting in pistons.

The presence of water also encourages diesel bug growth. This either occurs at the interface between the oil and water or on the tank, fuel pipe, and fuel filter walls, depending on whether or not the ‘bugs’ need oxygen.

What is Diesel Bug?

Diesel bug is the contamination of diesel fuel by microbes including over one hundred species of bacteria and fungus. Contrary to the commonly used terminology, these infections do not contain algae. Algae requires light for photosynthesis and there is no light in a diesel tank.

Diesel fuel itself is quite biodegradable. Given a very small amount of water, microbes will thrive and break the diesel down. Biodiesel is much more biodegradable. It is also host to well over one thousand species of bacteria and fungus. With the exposure to bacterial and fungal spores from purchased fuel, or even from the atmosphere, troublesome volumes of diesel bug can grow quite quickly. This is particularly true in warm weather and with some gentle agitation as your boat moves at the dock or swings at its mooring.

Two common diesel bugs: The bacteria Clostridium and the fungus Aspergillus. Source: Wikimedia

What Does Diesel Bug Do?

By breaking down the diesel molecules the microbes can cause a change in fuel chemistry, which adversely affects how the fuel is burned in your engine. On a rough day out on the water, clumps of these microbes can break loose and block fuel filters or worse the fuel injectors. Microbes in water can also cause the paper-element filters in the system to rot and fail, in turn causwing failure of the fuel pump or blockage of the injectors due to ingestion of large particles.

What Does It Look Like?

Diesel bug growth is recognizable as a brown, grey, or black gooey substance that coats the inside of the tank, pipes, and fuel filter elements. If the sludge collecting on your fuel filter consists of fine particulates, rust, or tarry material that leaves a black, sticky residue on your fingers, you're not looking at biological contamination. If the material is slimy you have a biological infection.

What Can You Do About Diesel Bug?

Filtration can help at low levels of infestation. The filters will catch the larger clumps of bacteria. However, even the finest fuel filter will not catch individual bacteria and spores, which measure less than 0.1 micron. The bacteria will pass through the filter unhindered where they can cause further trouble.


The only cure for a (serious) microbial infection is to kill it with biocides. 

Fuel companies agree that if left untreated, fuel will remain reliable for just 6–12 months. After which fuel contamination (such as the diesel bug) begins to appear. Most industrial engine manufacturers now recommend a fuel conditioning programme to ensure the reliability of fuel.

Some, like Yanmar, will suggest that treating a fuel tank with biocides can be dangerous and damaging to an engine. It is not the biocides that are damaging, but rather the dead bacteria and fungi clumps that suddenly release from the surfaces after treatment. This surge of slime can clog filters and can cause fuel starvation and other damage. If your tank was never previously treated, you will need to polish your fuel by pumping it through filters (see note on polishing fuel). You may also need to clean your tank.

For more information on diesel bug treatments, Practical Boat Owner did a test of 12 biocides in 2016, which remains very current:

How Does One Minimise the Risk of Fuel Contamination Occurring?

As with most things on a boat, prevention is better than cure. The key to maintaining fuel quality and preventing fuel contamination is good housekeeping. Good fuel housekeeping is a combination of effective processes, a little technology, and some good old-fashioned common sense.

·         Keep fuel tanks as full as possible to reduce water condensation from moisture-laden air

·         Use tank drains to periodically remove tank bottom water and sediment

·         If storing fuel in multiple tanks, employ a rotation system to use the oldest fuel first

·         Regularly check fuel for water, particulate and microbial contamination

·         If contamination is found, thoroughly clean both fuel and tanks

·         Regularly run a fuel polishing system to remove water and other contamination

·         Regularly add a fuel additive and biocide to maintain fuel stability and minimise microbial growth

·         Use only original equipment manufacturer (OEM) filters as the primaries.

·         Go for the finest (2 micron) filter cartridge on the (Racor) pre-filter as opposed to anything coarser


Fuel Polishing (Cleaning)

There are several commercially available fuel polishing systems. On our boat we built one ourselves. The key components are an electric fuel pump, two parallel Racor fuel pre-filters, and several three-way valves. In addition, we have two fuel tanks, though this is not necessary.

In the configuration shown here, we are pumping the fuel from one tank to the other passing through one of the Racor filters: thus, polishing the fuel.

When running the engine, we would direct the fuel flow to the engine with the return going to the tank from which it originated. The parallel filters allow us to “hot swap” the pre-filters, should one become blocked. The electric fuel pump has the additional advantage of making bleeding the fuel lines extremely easy after work was done, for example after changing the primary fuel filters.

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